1. Field of the Invention
The present invention relates to a sound recording apparatus and method.
2. Description of the Related Art
Recent photographing apparatus such as a camera are gaining more sophisticated functions, and many cameras have an advanced function capable of shooting both moving and still images. In moving image shooting, the photographing apparatus acquires a sound at the same time as acquiring an image, and records it in synchronism with the image.
However, the photographing apparatus such as a camera includes driving sources to adjust the optical system and the like, and the driving sounds from these driving sources are recorded as noise. Even the sound of a manipulation by the user propagates as vibrations through the housing of the photographing apparatus, and is recorded as noise. To solve these problems, various sound processing circuits and photographing apparatus have been proposed.
Japanese Patent Laid-Open No. 2008-053802 discloses a method of masking the head unloading sound of a hard disk. More specifically, a sound signal at a portion where the head unloading sound of the hard disk was generated is generated using preceding and succeeding reference sounds, thereby masking the head unloading sound. ITU-T Recommendation G.711-Appendix I describes a method of concealing a packet loss in sound transmission as a technique regarding transmission and demodulation of a sound. More specifically, a signal at a portion suffering a packet loss is generated by prediction using a reference sound signal before the loss, thereby concealing the packet loss.
In the conventional technique disclosed in Japanese Patent Laid-Open No. 2008-053802, a predicted sound is generated by performing calculation to predict a sound signal in a noise-generated section using, as reference sounds, object sounds before and after the noise-generated section. The generated predicted sound replaces a sound in the noise-generated section. The predicted sound is generated by, for example, successively predicting the next sample of a sound signal using the continuity of the sound waveform. Alternatively, detecting the repetition frequency of a sound and repeating the waveform in a predetermined cycle generate the predicted sound. This means that the precision of the reference sound needs to be high to generate a predicted sound with high precision. However, the precision of reference sounds decreases before and after the noise-generated section. This problem will be explained.
A general recorder includes a gain control circuit called an auto level control (ALC) circuit. When the object sound is loud, the ALC function decreases the amplification gain of a postamplifier for the sound detected by a microphone and records the sound, thereby preventing saturation of the sound signal. When the object sound is soft, the ALC function increases the amplification gain of the postamplifier for the sound detected by the microphone and records the sound, thereby increasing the S/N ratio of the sound signal. In this way, the ALC function finely changes the gain of a sound signal detected by the microphone in accordance with the loudness (sound pressure level) of the object sound. If the ALC function operates in the above-mentioned reference sounds generated section, the continuity of the sound pressure with a preceding object sound is lost, decreasing the precision of the predicted sound. That is, when the next sample of a sound signal is successively predicted using the continuity of the sound waveform, the continuity of the sound pressure is lost, failing prediction of the next sample. Also when the repetition frequency of a sound is detected to repeat the waveform in a predetermined cycle, detection of the repetition frequency of the sound fails. This results in poor precision of the predicted sound. This problem is not limited to when generating a predicted sound using reference sounds before and after the noise-generated section, as described in Japanese Patent Laid-Open No. 2008-053802. The precision of the predicted sound decreases even when the ALC amplification gain before a noise-generated section and that in the noise-generated section differ from each other in generating a predicted sound by synthesizing a reference sound before the noise-generated section and a sound in the noise-generated section. This also applies to a case wherein a sound in the noise-generated section is synthesized with a reference sound after a noise reduction section. This problem occurs not only when generating a predicted sound for noise removal processing, but also when, for example, a special effect is used to synthesize temporally preceding and succeeding object sounds. For example, the sound of a moving image is divided into a plurality of sections and the sections are synthesized. In this case, if the sections to be synthesized differ in ALC, their sound pressures become discontinuous, and no high-precision special effect (appropriate special effect) may be obtained.